Measuring body capacitance effect in touch sensitive device

ABSTRACT

Measuring an effect of body capacitance in a touch sensitive device is disclosed. This effect can be caused by poor grounding of a user or other objects touching the device or of the device itself. The device can operate in a stray capacitance mode to measure a body capacitance effect and in a normal mode to detect a touch on the device. During the stray capacitance mode, the device can obtain a body capacitance measurement from the device. During the normal mode, the device can obtain a touch measurement from the device. The device can calculate a body capacitance factor based on a ratio between the body capacitance measurement and the touch measurement and use the body capacitance factor to compensate for erroneous or distorted touch output values from the device. Various components of the device can be switchably configured according to the particular mode.

FIELD

This relates generally to touch sensitive devices and, moreparticularly, to measuring body capacitance affecting touch sensitivedevices.

BACKGROUND

Many types of input devices are presently available for performingoperations in a computing system, such as buttons or keys, mice,trackballs, joysticks, touch sensor panels, touch screens and the like.Touch sensitive devices, such as touch screens, in particular, arebecoming increasingly popular because of their ease and versatility ofoperation as well as their declining price. A touch sensitive device caninclude a touch sensor panel, which can be a clear panel with atouch-sensitive surface, and a display device such as a liquid crystaldisplay (LCD) that can be positioned partially or fully behind the panelso that the touch-sensitive surface can cover at least a portion of theviewable area of the display device. The touch sensitive device canallow a user to perform various functions by touching the touch sensorpanel using a finger, stylus or other object at a location dictated by auser interface (UI) being displayed by the display device. In general,the touch sensitive device can recognize a touch event and the positionof the touch event on the touch sensor panel, and the computing systemcan then interpret the touch event in accordance with the displayappearing at the time of the touch event, and thereafter can perform oneor more actions based on the touch event.

When either the object touching at the touch sensor panel or the touchsensitive device itself is poorly grounded, touch output values can beerroneous or otherwise distorted. More specifically, variouscapacitances, such as the object's body capacitance between the object,e.g., a finger, and ground can distort the capacitances measured at thetouch sensor panel, which can be utilized to generate the touch outputvalues. The possibility of such erroneous or distorted values canfurther increase when two or more simultaneous touch events occur at thetouch sensor panel.

SUMMARY

This relates to measuring an effect of body capacitance in a touchsensitive device due to poor grounding of a user or other objectstouching the device or of the device itself. The device can operate in astray capacitance mode to measure a body capacitance effect and in anormal mode to detect a touch on the device. The stray capacitance modeand the normal mode can operate concurrently. In addition oralternatively, the stray capacitance mode and the normal mode canoperate separately. During the stray capacitance mode, the device canobtain a body capacitance measurement from the device. During the normalmode, the device can obtain a touch measurement from the device. Thedevice can calculate a body capacitance factor based on a ratio betweenthe body capacitance measurement and the touch measurement and use thebody capacitance factor to compensate for erroneous or distorted touchoutput values from the device. Various components of the device can beconfigured to switch between the two modes.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an exemplary computing system that can measure aneffect of body capacitance according to various embodiments.

FIG. 2 illustrates an exemplary touch sensor panel in a no-touchcondition according to various embodiments.

FIG. 3 illustrates an exemplary touch map of a touch sensor panel in ano-touch condition according to various embodiments.

FIG. 4 illustrates an exemplary touch sensor panel at which there aremultiple touches from a well grounded user according to variousembodiments.

FIG. 5 illustrates an exemplary touch map of a touch sensor panel atwhich there are multiple touches from a well grounded user according tovarious embodiments.

FIG. 6 illustrates an exemplary touch sensor panel at which there aremultiple touches from a poorly grounded user according to variousembodiments.

FIG. 7 illustrates an exemplary touch map of a touch sensor panel atwhich there are multiple touches from a poorly grounded user accordingto various embodiments.

FIG. 8 illustrates an exemplary circuit that can measure an effect ofbody capacitance according to various embodiments.

FIG. 9 illustrates an exemplary method for calculating a factorrepresentative of a body capacitance effect according to variousembodiments.

FIG. 10 illustrates exemplary circuitry of a touch sensitive device instray capacitance mode for measuring an effect of body capacitanceaccording to various embodiments.

FIG. 11 illustrates an exemplary circuit in normal mode for detecting acapacitive touch according to various embodiments.

FIG. 12 illustrates an exemplary circuit in stray capacitance mode formeasuring an effect of body capacitance according to variousembodiments.

FIG. 13 illustrates an exemplary method for measuring an effect of bodycapacitance according to various embodiments.

FIG. 14 illustrates another exemplary circuit in stray capacitance modefor measuring an effect of body capacitance according to variousembodiments.

FIG. 15 illustrates another exemplary method for measuring an effect ofbody capacitance according to various embodiments.

FIG. 16 illustrates an exemplary circuit switchable between normal modeand stray capacitance mode according to various embodiments.

FIG. 17 illustrates an exemplary method for using a measured effect ofbody capacitance to compensate for negative capacitance according tovarious embodiments.

FIG. 18 illustrates an exemplary mobile telephone that can measure abody capacitance effect according to various embodiments.

FIG. 19 illustrates an exemplary digital media player that can measure abody capacitance effect according to various embodiments.

FIG. 20 illustrates an exemplary personal computer that can measure abody capacitance effect according to various embodiments.

DETAILED DESCRIPTION

In the following description of various embodiments, reference is madeto the accompanying drawings which form a part hereof, and in which itis shown by way of illustration specific embodiments which can bepracticed. It is to be understood that other embodiments can be used andstructural changes can be made without departing from the scope of thevarious embodiments.

This relates to measuring an effect of body capacitance in a touchsensitive device due to poor grounding of a user or other objectstouching the device or of the device itself. The device can operate in astray capacitance mode to measure a body capacitance effect and in anormal mode to detect a touch on the device. In some embodiments, thetwo modes can operate concurrently. In some embodiments, the two modescan operate alternately. In some embodiment, one mode can operatemultiple times, following by the other mode. During the straycapacitance mode, the device can obtain a body capacitance measurementfrom the device. During the normal mode, the device can obtain a touchmeasurement from the device. The device can calculate a body capacitancefactor based on a ratio between the body capacitance measurement and thetouch measurement and use the body capacitance factor to compensate forerroneous or distorted touch output values from the device. Variouscomponents of the device can be switchably configured according to theparticular mode.

The ability to measure an effect of body capacitance in a touchsensitive device can advantageously compensate touch output values forunwanted touch artifacts, thereby providing more accurate and fastertouch detection by not having to repeat measurement subject to poorgrounding conditions. Power savings can also be realized by not havingto repeat measurements. Additionally, the device can more robustly adaptto various grounding conditions of a user.

The terms “poorly grounded,” “ungrounded,” “not grounded,” “not wellgrounded,” “improperly grounded,” “isolated,” and “floating” can be usedinterchangeably to refer to poor grounding conditions that can existwhen an object is not making a low resistance electrical connection tothe ground of the touch sensitive device.

The terms “grounded,” “properly grounded,” and “well grounded” can beused interchangeably to refer to good grounding conditions that canexist when an object is making a low resistance electrical connection tothe ground of the touch sensitive device.

Although various embodiments can be described and illustrated herein interms of mutual capacitance touch sensor panels, it should be understoodthat the various embodiments are not so limited, but can be additionallyapplicable to self-capacitance sensor panels, and both single andmulti-touch sensor panels, and other sensors, in which singlestimulation signals can be used to generate a touch signal and in whichmultiple simultaneous stimulation signals can be used to generate acomposite touch signal. Furthermore, although various embodiments can bedescribed and illustrated herein in terms of double-sided ITO (DITO)touch sensor panels, it should be understood that the variousembodiments can be also applicable to other touch sensor panelconfigurations, such as configurations in which the drive and senselines can be formed on different substrates or on the back of a coverglass, and configurations in which the drive and sense lines can beformed on the same side of a single substrate.

FIG. 1 illustrates an exemplary computing system 100 that can measure aneffect of body capacitance according to various embodiments describedherein. In the example of FIG. 1, computing system 100 can include touchcontroller 106. The touch controller 106 can be a single applicationspecific integrated circuit (ASIC) that can include one or moreprocessor subsystems 102, which can include one or more main processors,such as ARM968 processors or other processors with similar functionalityand capabilities. However, in other embodiments, the processorfunctionality can be implemented instead by dedicated logic, such as astate machine. The processor subsystems 102 can also include peripherals(not shown) such as random access memory (RAM) or other types of memoryor storage, watchdog timers and the like. The touch controller 106 canalso include receive section 107 for receiving signals, such as touchsignals 103 of one or more sense channels (not shown), other signalsfrom other sensors such as sensor 111, etc. The touch controller 106 canalso include demodulation section 109 such as a multistage vectordemodulation engine, panel scan logic 110, and transmit section 114 fortransmitting stimulation signals 116 to touch sensor panel 124 to drivethe panel. The panel scan logic 110 can access RAM 112, autonomouslyread data from the sense channels, and provide control for the sensechannels. In addition, the panel scan logic 110 can control the transmitsection 114 to generate the stimulation signals 116 at variousfrequencies and phases that can be selectively applied to rows of thetouch sensor panel 124.

The touch controller 106 can also include charge pump 115, which can beused to generate the supply voltage for the transmit section 114. Thestimulation signals 116 can have amplitudes higher than the maximumvoltage by cascading two charge store devices, e.g., capacitors,together to form the charge pump 115. Therefore, the stimulus voltagecan be higher (e.g., 6V) than the voltage level a single capacitor canhandle (e.g., 3.6 V). Although FIG. 1 shows the charge pump 115 separatefrom the transmit section 114, the charge pump can be part of thetransmit section.

Touch sensor panel 124 can include a capacitive sensing medium havingrow traces (e.g., drive lines) and column traces (e.g., sense lines),although other sensing media can also be used. The row and column tracescan be formed from a transparent conductive medium such as Indium TinOxide (ITO) or Antimony Tin Oxide (ATO), although other transparent andnon-transparent materials such as copper can also be used. In someembodiments, the row and column traces can be perpendicular to eachother, although in other embodiments other non-Cartesian orientationsare possible. For example, in a polar coordinate system, the sense linescan be concentric circles and the drive lines can be radially extendinglines (or vice versa). It should be understood, therefore, that theterms “row” and “column” as used herein are intended to encompass notonly orthogonal grids, but the intersecting traces of other geometricconfigurations having first and second dimensions (e.g. the concentricand radial lines of a polar-coordinate arrangement). The rows andcolumns can be formed on, for example, a single side of a substantiallytransparent substrate separated by a substantially transparentdielectric material, on opposite sides of the substrate, on two separatesubstrates separated by the dielectric material, etc.

At the “intersections” of the traces, where the traces pass above andbelow (cross) each other (but do not make direct electrical contact witheach other), the traces can essentially form two electrodes (althoughmore than two traces can intersect as well). Each intersection of rowand column traces can represent a capacitive sensing node and can beviewed as picture element (pixel) 126, which can be particularly usefulwhen the touch sensor panel 124 is viewed as capturing an “image” oftouch. (In other words, after the touch controller 106 has determinedwhether a touch event has been detected at each touch sensor in thetouch sensor panel, the pattern of touch sensors in the multi-touchpanel at which a touch event occurred can be viewed as an “image” oftouch (e.g. a pattern of fingers touching the panel).) The capacitancebetween row and column electrodes can appear as a stray capacitanceCstray when the given row is held at direct current (DC) voltage levelsand as a mutual signal capacitance Csig when the given row is stimulatedwith an alternating current (AC) signal. The presence of a finger orother object near or on the touch sensor panel can be detected bymeasuring changes to a signal charge Qsig present at the pixels beingtouched, which can be a function of Csig. The signal change Qsig canalso be a function of a capacitance Cbody of the finger or other objectto ground, as will be described in more detail later.

Computing system 100 can also include host processor 128 for receivingoutputs from the processor subsystems 102 and performing actions basedon the outputs that can include, but are not limited to, moving anobject such as a cursor or pointer, scrolling or panning, adjustingcontrol settings, opening a file or document, viewing a menu, making aselection, executing instructions, operating a peripheral deviceconnected to the host device, answering a telephone call, placing atelephone call, terminating a telephone call, changing the volume oraudio settings, storing information related to telephone communicationssuch as addresses, frequently dialed numbers, received calls, missedcalls, logging onto a computer or a computer network, permittingauthorized individuals access to restricted areas of the computer orcomputer network, loading a user profile associated with a user'spreferred arrangement of the computer desktop, permitting access to webcontent, launching a particular program, encrypting or decoding amessage, and/or the like. The host processor 128 can also performadditional functions that may not be related to panel processing, andcan be coupled to program storage 132 and display device 130 such as anLCD display for providing a UI to a user of the device. In someembodiments, the host processor 128 can be a separate component from thetouch controller 106, as shown. In other embodiments, the host processor128 can be included as part of the touch controller 106. In still otherembodiments, the functions of the host processor 128 can be performed bythe processor subsystem 102 and/or distributed among other components ofthe touch controller 106. The display device 130 together with the touchsensor panel 124, when located partially or entirely under the touchsensor panel or when integrated with the touch sensor panel, can form atouch sensitive device such as a touch screen.

A body capacitance effect can be determined by the processor insubsystem 102, the host processor 128, dedicated logic such as a statemachine, or any combination thereof according to various embodiments.

Note that one or more of the functions described above can be performed,for example, by firmware stored in memory (e.g., one of the peripherals)and executed by the processor subsystem 102, or stored in the programstorage 132 and executed by the host processor 128. The firmware canalso be stored and/or transported within any computer readable storagemedium for use by or in connection with an instruction execution system,apparatus, or device, such as a computer-based system,processor-containing system, or other system that can fetch theinstructions from the instruction execution system, apparatus, or deviceand execute the instructions. In the context of this document, a“computer readable storage medium” can be any medium that can contain orstore the program for use by or in connection with the instructionexecution system, apparatus, or device. The computer readable storagemedium can include, but is not limited to, an electronic, magnetic,optical, electromagnetic, infrared, or semiconductor system, apparatusor device, a portable computer diskette (magnetic), a random accessmemory (RAM) (magnetic), a read-only memory (ROM) (magnetic), anerasable programmable read-only memory (EPROM) (magnetic), a portableoptical disc such a CD, CD-R, CD-RW, DVD, DVD-R, or DVD-RW, or flashmemory such as compact flash cards, secured digital cards, USB memorydevices, memory sticks, and the like.

The firmware can also be propagated within any transport medium for useby or in connection with an instruction execution system, apparatus, ordevice, such as a computer-based system, processor-containing system, orother system that can fetch the instructions from the instructionexecution system, apparatus, or device and execute the instructions. Inthe context of this document, a “transport medium” can be any mediumthat can communicate, propagate or transport the program for use by orin connection with the instruction execution system, apparatus, ordevice. The transport medium can include, but is not limited to, anelectronic, magnetic, optical, electromagnetic or infrared wired orwireless propagation medium.

It is to be understood that the touch sensor panel is not limited totouch, as described in FIG. 1, but can be a proximity panel or any otherpanel according to various embodiments. In addition, the touch sensorpanel described herein can be either a single-touch or a multi-touchsensor panel.

It is further to be understood that the computing system is not limitedto the components and configuration of FIG. 1, but can include otherand/or additional components in various configurations capable ofmeasuring an effect of body capacitance according to variousembodiments.

FIG. 2 illustrates an exemplary touch sensor panel in a no-touchcondition, i.e., where there are no present touches at the panel,according to various embodiments. In the example of FIG. 2, touch sensorpanel 124 can include an array of pixels 126 that can be formed at thecrossings of rows of drive lines 201 (D0-D3) and columns of sense lines202 (S0-S3). Each pixel 126 can have an associated mutual capacitanceCsig when the drive line 201 forming the pixel is stimulated with astimulation signal Vstm 116. Each pixel 126 can also have an associatedstray capacitance Cstray when the drive line 201 forming the pixel isnot stimulated with a stimulation signal Vstm 116 but is connected toDC. In this example, drive line D0 can be stimulated with stimulationsignal 116 (Vstm), forming mutual capacitance Csig at the pixels 126formed between the drive line D0 and the crossing sense lines S0-S3.

FIG. 3 illustrates an exemplary touch map of a touch sensor panel in ano-touch condition according to various embodiments. In the example ofFIG. 3, drive lines D0-D3 of touch sensor panel 124 can be individuallyand/or simultaneously stimulated with stimulation signal(s) Vstm 116.Since there are no fingers touching at the pixels 126 formed by thedrive lines D0-D3 and the crossing sense lines S0-S3, the fingers cannotblock some of the electric field lines formed when the drive lines arestimulated so as to reduce the mutual capacitance Csig by an amountΔCsig. As such, the touch map can remain substantially flat at thepixels 126 to indicate no touch.

FIG. 4 illustrates an exemplary touch sensor panel at which there arepresently multiple touches from a well grounded user according tovarious embodiments. In the example of FIG. 4, one of drive lines 401(D0) of touch sensor panel 124 can be stimulated with a stimulationsignal 116 (Vstm) to form a mutual capacitance Csig between thestimulated drive line D0 and the crossing sense lines 402 (S0-S3). Inthis example, user 419 can touch with finger 419-a at pixel 126 formedby drive line D0 and sense line S1 (“pixel D0,S1”) and with finger 419-bat pixel 126 formed by drive line D1 and sense line S2 (“pixel D1,S2”).The user's body can have a capacitance Cbody to ground, e.g., about 200pF in some embodiments. When the user's finger 419-a touches at thetouch sensor panel 124, the finger can block some of the electric fieldlines formed when the drive line D0 is stimulated and those electricfield lines can be shunted to ground through the capacitance path 408from the finger through the body to ground. As a result, the mutualcapacitance Csig at pixel D0,S1 can be reduced by ΔCsig. In thisexample, since drive line D1 is not being stimulated with Vstm while D0is being stimulated, the finger 419-b at pixel D1,S2 cannot affect thepixel's capacitance.

Similar to the finger 419-a, when drive line D1 is stimulated with Vstm,the finger 419-b can reduce the mutual capacitance Csig by ΔCsig atpixel D1,S2. In this example, since drive line D0 is not beingstimulated with Vstm while D1 is being stimulated, the finger 419-a atpixel D0,S1 cannot affect the pixel's capacitance.

FIG. 5 illustrates an exemplary touch map of a touch sensor panel atwhich there are presently multiple touches from a well grounded useraccording to various embodiments. In the example of FIG. 5, drive linesD0-D3 of touch sensor panel 124 can be individually and/orsimultaneously stimulated with stimulation signal(s) Vstm 116. User 419can touch with finger 419-a at pixel D0,S1 and with finger 419-b atpixel D1,S2, as in FIG. 4. When drive line D0 is stimulated with Vstm,the finger 419-a at pixel D0,S1 can block some of the electric fieldlines formed during the stimulation, thereby reducing Csig by ΔCsig atpixel D0,S1. As such, the touch map can indicate a true touch of thefinger 419-a at pixel D0,S1 (symbolically illustrated by a peak) whendrive line D0 is stimulated. Similarly, when drive line D1 is stimulatedwith Vstm, the finger 419-b at pixel D1,S2 can block some of theelectric field lines formed during the stimulation, thereby reducingCsig by ΔCsig at pixel D1,S2. As such, the touch map can indicate a truetouch of the finger 419-b at pixel D1,S2 (symbolically illustrated by apeak) when drive line D1 is stimulated.

The touch map in this example indicates the touches when both drivelines D0 and D1 are stimulated. However, in the case where only driveline D0 is stimulated, the touch map can indicate a touch of the finger419-a at pixel D0,S1, but not a touch of the finger 419-b at pixelD1,S2. Conversely, in the case where only drive line D1 is stimulated,the touch map can indicate a touch of the finger 419-b at pixel D1,S2,but not a touch of the finger 419-a at pixel D0,S1.

FIG. 6 illustrates an exemplary touch sensor panel at which there aremultiple touches from a poorly grounded user according to variousembodiments. In the example of FIG. 6, one of drive lines 601 (D0) oftouch sensor panel 124 can be stimulated with a stimulation signal 116(Vstm) to form a mutual capacitance Csig between the stimulated driveline D0 and the crossing sense lines 602 (S0-S3). In this example, user619 can touch with finger 619-a at pixel 126 formed by drive line D0 andsense line S1 (“pixel D0,S1”) and with finger 619-b at pixel 126 formedby drive line D1 and sense line S2 (“pixel D1,S2”). Because the user 619is poorly grounded, the user's body capacitance Cbody to ground can varysignificantly, e.g., between 10-100 pF in some embodiments. The user'sfinger 619-a can also form a capacitance Cfd from the stimulated driveline D0 to that finger. When the finger 419-a touches at the touchsensor panel 124, the finger can block some of the electric field linesformed when the drive line D0 is stimulated. But instead of thoseelectric field lines being shunted to ground, a capacitance Cfs from thefinger 619-a to the sense line S1 can form, sending some of the chargefrom the electric field lines through capacitive path 608-a from thefinger to the sense line S1. As a result, instead of the mutualcapacitance Csig at pixel D0,S1 being reduced by ΔCsig, Csig can only bereduced by (ΔCsig−Cneg), where Cneg can represent a so-called “negativecapacitance” resulting from the charge sent into sense line S1 due tothe poor grounding of the user 419. Negative capacitance will bedescribed in more detail below.

Similarly, a capacitance Cfs from the finger 619-b to the sense line S2can also form, sending some of the charge from the electric field linesthrough capacitive path 608-b from the finger to the sense line S2. As aresult, though drive line D1 is not being stimulated with Vstm whiledrive line D0 is being stimulated, the finger 619-b at pixel D1,S2 canincrease the pixel's capacitance by Cneg (a negative capacitance at thepixel resulting from the charge sent into sense line S2 due to the poorgrounding of the user) to a capacitance above that of no touch to givethe appearance of a so-called “negative pixel” or a theoretical negativeamount of touch at pixel D1,S2.

Adjacent pixels can also experience this negative pixel effect. Thepixel 126 formed by the drive line D0 and the crossing sense line S2(“pixel D0,S2”) can increase the pixel's capacitance Csig by Cneg, dueto the capacitance Cfs introduced by the finger 619-b into the senseline S2. Similarly, the pixel 126 formed by the drive line D1 and thecrossing sense line S1 (“pixel D1,S1”) can increase the pixel'scapacitance by Cneg to a capacitance above that of no touch, due to thecapacitance Cfs introduced by the finger 619-a into the sense line S1.

Similar to the finger 619-a, when drive line D1 is stimulated with Vstm,the finger 619-b can reduce the mutual capacitance Csig by (ΔCsig-Cneg)at pixel D1,S2, where Cneg can be a negative capacitance at the pixelresulting from the charge from the electric field lines sent into senseline S2 when drive line D1 is stimulated. In this example, since driveline D0 is not stimulated while drive line D1 is stimulated, the finger619-a at pixel D0,S1 can increase the pixel's capacitance by Cneg togive the appearance of a negative pixel. Adjacent pixels D1,S2 and D0,S1can similarly experience the negative pixel effect.

FIG. 7 illustrates an exemplary touch map of a touch sensor panel wherethere are presently multiple touches from a poorly grounded useraccording to various embodiments. In the example of FIG. 7, drive linesD0-D3 of touch sensor panel 124 can be individually and/orsimultaneously stimulated with stimulation signal(s) Vstm 116. User 619can touch with finger 619-a at pixel D0,S1 and with finger 619-b atpixel D1,S2, as in FIG. 6. When drive line D0 is stimulated, the finger619-a at pixel D0,S1 can shunt some of the blocked electric field linesinto the sense line S1 instead of into ground, such that the touch mapcan indicate an attenuated true touch at that pixel (symbolicallyillustrated by a slight peak). Similarly, when drive line D1 isstimulated, the finger 619-b at pixel D1,S2 can shunt some of theblocked electric field lines into the sense line S2 instead of intoground, such that the touch map can indicate an attenuated true touch atthat pixel (symbolically illustrated by a slight peak). Adjacent pixelsD1,S1 and D0,S2 can experience a negative pixel effect, as describedpreviously, such that the touch map can indicate a negative touch atthese pixels (symbolically illustrated by hollows). The net result ofthe user being poorly grounded can be that the touch signal of the pixelbeing touched can be attenuated and the adjacent pixels can experience anegative pixel effect.

The touch map in this example indicates the touches when both drivelines D0 and D1 are stimulated. However, in the case where only driveline D0 is stimulated, the touch map can indicate an attenuated touch ofthe finger 619-a at pixel D0,S1 and negative touches of variousmagnitudes of the finger 619-b at pixel D1,S2 and at adjacent no-touchpixels D1,S1 and D0,S2. Conversely, in the case where only drive line D1is stimulated, the touch map can indicate an attenuated touch of thefinger 619-b at pixel D1,S2 and negative touches of various magnitudesof the finger 619-a at pixel D0,S1 and at adjacent no-touch pixels D1,S1and D0,S2.

As previously described, negative capacitance can distort pixel touchoutput values. The extent to which negative capacitance occurs can be afunction of the extent to which the user is grounded, which can bereflected in the user's body capacitance Cbody. As such, measuring aneffect of the body capacitance and compensating for the negative pixeleffect based on that measurement can eliminate or at least attenuatedistortion in the pixel touch output values.

In a no-touch condition as illustrated in FIG. 2, for example, theresulting pixel touch output value Vout at the output of atransimpedence amplifier (such as amplifier 809 in FIG. 8) of a sensechannel can be a function of Vstm, the stimulation signal to the driveline crossing the sense line outputting to the transimpedence amplifier;Csig, the mutual capacitance between the drive line and the sense line;Rfb, the resistance of the transimpedence amplifier's feedback resistor;and the stimulation signal frequency, for example.

When a well grounded object, such as a user's finger, touches at thetouch sensor panel as illustrated in FIG. 4, for example, the mutualcapacitance Csig can be decreased by ΔCsig and the no-touch pixel outputvalue Vout can be reduced by Vs. When a poorly grounded object, such asa user's finger, touches at the touch sensor panel as illustrated inFIG. 6, for example, undesirable charge coupling called negativecapacitance Cneg can be introduced into the panel. The negativecapacitance Cneg can include a finger to drive line capacitance Cfd inseries with a finger to sense line capacitance Cfs. There can also be aground capacitance Cgnd, which can be a function of the devicecapacitance Cd and the user's body capacitance Cbody and can beexpressed as the parallel sum of the capacitance Cbody between theuser's body and ground and the capacitance Cd between the device chassisand ground. The negative capacitance Cneg can be equivalent to thecombination of Cfd, Cfs, and Cgnd.

When an object is well grounded, Cgnd can be a large value relative toCfd and Cfs, such that Cneg can be negligible. Additionally, any Cfd canhave the positive effect of increasing the drive current in the driveline being stimulated and any Cfs can have the positive effect of beingshunted by the virtual ground of the transimpedence amplifier.

In contrast, when an object is poorly grounded, Cfs, Cfd, and Cgnd canbe on the same order of magnitude. The negative capacitance can causethe voltage detected by the transimpedence amplifier to be higher by anamount Vn than under well grounded conditions. The resulting pixel touchoutput value Vout” can be the no-touch pixel output value Vout reducedby Vs and increased by Vn. The negative capacitance effect (or bodycapacitance effect) on the actual pixel touch output value can be in theopposite direction of the intended touch capacitance change. As such, apixel experiencing touch under poor grounding conditions can detect lessof a touch than is actually present.

FIG. 8 illustrates an exemplary circuit that can measure an effect ofbody capacitance according to various embodiments. In the example ofFIG. 8, touch sensitive device circuit 800 can operate in a normal mode,during which the circuit can detect a capacitive touch at the device,and in a stray capacitance mode, during which the circuit can detect abody capacitance effect in the device. The circuit 800 can includetransmit section 114, touch sensor panel 124, and receive section 107.FIG. 8 shows the details of one drive channel of the transmit section114 and one sense channel of the receive section 107. However, it is tobe understood that multiple drive channels and sense channels havingsimilar components can be used.

The transmit section 114 can include transmit digital-to-analogconverter (DAC) 801. The transmit DAC 801 can convert digital signalsfrom transmit logic into stimulation signals Vstm_p and Vstm_n to supplyto the drive channels. Vstm_p can be a positive (+) phase signal. Vstm_ncan be a negative (−) phase signal, inverted relative to Vstm_p about acommon voltage Vcm. Vstm_p and Vstm_n can have variable amplitude andcan have sinusoidal or other wave shapes (e.g., ramped square wave,etc.). Stimulation signal Vstm_sc_p, a positive (+) phase signal, canalso supply the drive channels in stray capacitance mode. Vstm_sc_p canbe scaled from Vstm_n and generated via buffer 808-a. In someembodiments, stimulation signal Vstm_sc_n, a negative (−) phase signalinverted relative to Vstm_sc_p, can also supply the drive channels instray capacitance mode. Vstm_sc_n can be scaled from Vstm_p andgenerated via buffer 808-b. The transmit section 114 can also connectthe drive channels to Vcm or to ground (gnd). A drive channel of thetransmit section 114 can include multiplexer 803 and output buffer 805.The multiplexer 803 of the drive channel can select one of the drivesignals, Vstm_p, Vstm_n, Vstm_sc_p, Vstm_sc_n, Vcm, or gnd, to supplythe corresponding buffer 805. The multiplexer 803 can select the drivesignal based on transmit select signal 802. When the circuit is innormal mode, the select signal 802 can select any of the drive signals,depending on the particular stimulation pattern to be applied to thetouch sensor panel 124 to stimulate the drive lines for touch or neartouch detection. When the circuit is in stray capacitance mode, theselect signal 802 can select any of the drive signals, depending on theparticular manner in which the body capacitance effect is measured, aswill be described in more detail below. The buffer 805 of the drivechannel can increase the gain of the signal from the transmit DAC 801and provide the drive capability to drive the mostly capacitive loadpresented to the buffer 805 by the touch sensor panel.

A sense channel of the receive section 107 can include receivemultiplexer 807 and transimpedence amplifier 809. Other amplifiers canalso be used, e.g., a charge amplifier. The transimpedence amplifier 809can convert the touch signal current from the touch sensor panel 124into a voltage signal. The voltage signal can be fed to other componentsfor further processing. The multiplexer 807 can select either areference voltage Vref or a stimulation signal Vstm_sc_p or Vstm_sc_n tobe fed to the non-inverting input of the amplifier 809 in order toconvert the touch signal to a voltage. The multiplexer 807 can selectthe non-inverting input based on receive select signal 806. When thecircuit is in normal mode, the select signal 806 can select Vref forprocessing touch signals to indicate a touch event. When the circuit isin stray capacitance mode, the select signal 806 can select Vstm_sc_p orVstm_sc_n for measuring an effect of body capacitance to correct forpoor grounding conditions.

In some embodiments, in stray capacitance mode, Vstm_sc_p or Vstm_sc_nfrom the multiplexer 807 can be transmitted to other components, e.g.,to a downstream bandpass filter, to improve the dynamic range at theoutput of the transimpedence amplifier 809. When either stimulationsignal Vstm_sc_p or Vstm_sc_n supplies the non-inverting input of thetransimpedence amplifier 809, the output voltage of the amplifier canbecome a function of the stray capacitance Cstray coupled to theinverting input of the amplifier, such that the stimulation signalVstm_sc appears at the inverting input as the amplifier tries tomaintain equilibrium at both inverting and non-inverting inputs. Cstraycan represent the sum of all the mutual capacitances Csig on thatparticular channel and any parasitic capacitances to ground Cgnd. Thegain Gtia_sc of the amplifier 809 in stray capacitance mode can be, forexample,

G _(TIA) _(—) _(SC)=√{square root over (1+(ω_(STM) ·R _(fb) ·C_(STRAY))²)},   (1)

where ωstm is the stimulation frequency of Vstm_sc in radians and Rfb isthe feedback resistance. The “1” term under the square root in Equation(1) can be undesirable as not providing any information about the actualsignal out of the amplifier and as limiting the amplifier's dynamicoutput range by about 6 dB or more, depending on the stimulationfrequency. By removing, e.g., subtracting, the stimulation signalVstm_sc from the output of the amplifier 809, the undesirable “1” termcan be removed, such that the gain Gtia_sc can become, for example,

G _(TIA) _(—) _(SC)=ω_(STM) ·R _(fb) ·C _(STRAY).   (2)

In some embodiments, the removal of the stimulation signal Vstm_sc canbe effected by transmitting the stimulation signal Vstm_sc from themultiplexer 807 to a downstream bandpass filter (or some otherprocessing component) as a reference signal.

In addition to gaining dynamic range, removing the stimulation signalVstm_sc from the output of the amplifier 809 can further result in thephase shift between the stimulation signal Vstm_sc and the amplifieroutput voltage Vout being approximately constant. For example, the phaseshift can become approximate π/2. As such, the dependency of theamplifier 809 on the stimulation signal frequency-dependent phase shiftcan be reduced.

The touch sensor panel 124 can receive stimulation signals from thetransmit section 114 and can output touch signals to the receive section107 in both modes.

The body capacitance effect can be approximately expressed in terms ofthe relationship between the measured touch output value ΔCsig,m and theactual touch output value ΔCsig,a at a pixel as follows,

$\begin{matrix}{{\Delta \; {C_{{sig},a}\left( {i,j} \right)}} = {{\Delta \; {C_{{sig},m}\left( {i,j} \right)}} + \frac{\sum\limits_{all\_ j}{{C_{fd}\left( {i,j} \right)} \times {\sum\limits_{all\_ i}{C_{fs}\left( {i,j} \right)}}}}{\begin{matrix}{{\sum\limits_{{all\_ j},{all\_ i}}{C_{fd}\left( {i,j} \right)}} +} \\{{\sum\limits_{{all\_ j},{all\_ i}}{C_{fs}\left( {i,j} \right)}} + C_{gnd}}\end{matrix}}}} & (3)\end{matrix}$

where (i,j)=the location of the pixel formed by the crossing of driveline i and sense line j in the touch sensor panel;

${{\sum\limits_{all\_ j}{C_{fd}\left( {i,j} \right)}} = {{the}\mspace{14mu} {sum}\mspace{14mu} {of}\mspace{14mu} {all}\mspace{14mu} {finger}\mspace{14mu} {to}\mspace{14mu} {drive}\mspace{14mu} {line}\mspace{14mu} {capacitances}\mspace{11mu} {Cfd}\mspace{14mu} {along}\mspace{14mu} {drive}\mspace{14mu} {line}\mspace{14mu} i}};$${{\sum\limits_{all\_ i}{C_{fs}\left( {i,j} \right)}} = {{the}\mspace{14mu} {sum}\mspace{14mu} {of}\mspace{14mu} {all}\mspace{14mu} {finger}\mspace{14mu} {to}\mspace{14mu} {sense}\mspace{14mu} {line}\mspace{14mu} {capacitances}\mspace{14mu} {Cfs}\mspace{14mu} {along}\mspace{14mu} {sense}\mspace{14mu} {line}\mspace{14mu} j}};$${{\sum\limits_{{all\_ j},{all\_ i}}{C_{fd}\left( {i,j} \right)}} = {{the}\mspace{14mu} {sum}\mspace{14mu} {of}\mspace{14mu} {all}\mspace{14mu} {finger}\mspace{14mu} {to}\mspace{14mu} {drive}\mspace{14mu} {line}\mspace{14mu} {capacitances}\mspace{14mu} {Cfd}\mspace{14mu} {in}\mspace{14mu} {the}\mspace{14mu} {touch}\mspace{14mu} {sensor}\mspace{14mu} {panel}}};$${\sum\limits_{{all\_ j},{all\_ i}}{C_{fs}\left( {i,j} \right)}} = {{the}\mspace{14mu} {sum}\mspace{14mu} {of}\mspace{14mu} {all}\mspace{14mu} {finger}\mspace{14mu} {to}\mspace{14mu} {sense}\mspace{14mu} {line}\mspace{14mu} {capacitances}}$

Cfs in the touch sensor panel; and Cgnd=ground capacitance, which can bea function of the device capacitance and the user's body capacitance,i.e., how well the user is grounded. The right-hand added term inEquation (3) can represent the body capacitance effect.

Equation (3) can be rearranged as follows,

$\begin{matrix}{{{\Delta \; {C_{{sig},a}\left( {i,j} \right)}} = {{\Delta \; {C_{{sig},m}\left( {i,j} \right)}} + {R \times {\sum\limits_{all\_ j}{\Delta \; {C_{{sig},m}\left( {i,j} \right)} \times {\sum\limits_{all\_ i}{\Delta \; {C_{{sig},m}\left( {i,j} \right)}}}}}}}},} & (4)\end{matrix}$

where R=a body capacitance factor, which can be a function of Cfd, Cfs,and Cgnd, thereby representative of a user's body capacitance Cbody;

${{\sum\limits_{all\_ j}{\Delta \; {C_{{sig},m}\left( {i,j} \right)}}} = {{the}\mspace{14mu} {sum}\mspace{14mu} {of}\mspace{14mu} {all}\mspace{14mu} {measured}\mspace{14mu} {touch}\mspace{14mu} {signal}\mspace{14mu} {outputs}\mspace{14mu} {along}\mspace{14mu} {drive}\mspace{14mu} {line}\mspace{14mu} i}};$${{and}\mspace{14mu} {\sum\limits_{all\_ i}{\Delta \; {C_{{sig},m}\left( {i,j} \right)}}}} = {{the}\mspace{14mu} {sum}\mspace{14mu} {of}\mspace{14mu} {all}\mspace{14mu} {measured}\mspace{14mu} {touch}\mspace{14mu} {signal}\mspace{14mu} {outputs}\mspace{14mu} {along}\mspace{14mu} {sense}\mspace{14mu} {line}\mspace{14mu} {j.}}$

The body capacitance factor R can be approximated as follows,

$\begin{matrix}{{R = {a \times \left( \frac{\sum{S(j)}}{\sum{Z_{m}(j)}} \right)_{all\_ j}}},} & (5)\end{matrix}$

where a=a touch sensor panel design constant, which can be obtainedthrough simulation and/or empirical measurements for a given panelsensing pattern design; S(j)=a cross product of the finger to sense linecapacitances Cfs along the sense lines obtained when a sense line j isstimulated with a stimulation signal Vstm_sc in a stray capacitanceconfiguration; and Zm(j)=an estimated cross product of the finger tosense line capacitances Cfs along the sense lines using the measuredtouch signal values ΔCsig,m obtained from the sense lines when the drivelines are stimulated in a normal configuration.

The cross product S(j) of the finger to sense line capacitances Cfs canbe obtained during stray capacitance mode by stimulating a sense line jof the touch sensor panel with a stimulation signal Vstm_sc and sensingtouch outputs on sense line j and a set of k unstimulated sense lines(where k=any number less than or equal to the number of unstimulatedsense lines), where the measurement S(j) can be expressed as follows,

$\begin{matrix}{{S(j)} = {\frac{\sum\limits_{all\_ i}{{C_{fs}\left( {i,j} \right)} \times {\sum\limits_{{all\_ i},{all\_ k}}{C_{fs}\left( {i,k} \right)}}}}{{\sum\limits_{{all\_ j},{all\_ i}}{C_{fd}\left( {i,j} \right)}} + {\sum\limits_{{all\_ j},{all\_ i}}{C_{fs}\left( {i,j} \right)}} + C_{gnd}}.}} & (6)\end{matrix}$

Similarly, the estimated cross product Zm(j) of the finger to sense linecapacitances Cfs using the measured touch output values ΔCsig,m can beobtained during normal mode by stimulating the drive lines of the touchsensor panel with a stimulation signal Vstm and sensing touch outputs onsense line j and the set of k other sense lines, where Zm(j) can beexpressed as follows,

$\begin{matrix}\begin{matrix}{{Z_{m}(j)} = {\sum\limits_{all\_ i}{{C_{{fs},m}\left( {i,j} \right)} \times {\sum\limits_{{all\_ i},{all\_ k}}{C_{{fs},m}\left( {i,k} \right)}}}}} \\{{= {b{\sum\limits_{all\_ i}{\Delta \; {C_{{sig},m}\left( {i,j} \right)} \times b{\sum\limits_{{all\_ i},{all\_ k}}{\Delta \; {C_{{sig},m}\left( {i,k} \right)}}}}}}},}\end{matrix} & (7)\end{matrix}$

where b=a touch sensor panel design constant, which can be obtainedthrough simulation and/or empirical measurements for a given panelsensing pattern design.

Hence, by obtaining S(j) and Zm(j) and multiplying their sum ratios bythe touch sensor panel design constant a as in Equation (5), the bodycapacitance factor R can be determined. R can then be used to compensatefor the body capacitance effect at a pixel, as in Equation (4).

FIG. 9 illustrates an exemplary method for measuring body capacitancefactor R according to various embodiments. In the example of FIG. 9,normal mode can be enabled (905). During normal mode, the drive channelscan output stimulation signals Vstm (e.g., Vstm_p or Vstm_n) and thenon-inverting inputs of the sense channels' transimpedence amplifierscan be connected to reference voltage Vref. A touch panel scan can beperformed (910). During the scan, the drive lines of the touch sensorpanel can be separately or simultaneously stimulated with stimulationsignals Vstm from the drive channels, and the sense lines of the panelcan output touch signals to the sense channels. The sense channeloutput(s) can be captured (915). The measurement Zm can be obtained fora particular sense line j (920). For example, the output from the sensechannel connected to sense line j and the outputs from k other sensechannels connected to k other sense lines can be summed to obtain Zm.Alternatively, the outputs can be averaged to obtain Zm.

Stray capacitance mode can be enabled (925). During stray capacitancemode, the drive channels can connect the touch sensor panel drive linesto either stimulation signals Vstm_sc (e.g., Vstm_sc_p or Vstm_sc_n) ora common voltage Vcm or ground. The non-inverting input of a sensechannel's transimpedence amplifier can be connected to the stimulationsignal Vstm_sc (e.g., Vstm_Sc_p or Vstm_sc_n) and the remaining sensechannels' amplifiers' non-inverting inputs can remain connected to Vref.Alternatively, multiple amplifiers can connect to Vstm_sc. A touch panelscan can be performed (935). During the scan, the sense line of thetouch sensor panel connected to the Vstm_sc-inputted sense channel canbe stimulated with the stimulation signal Vstm_sc and the remainingsense lines of the panel can output touch signals to the remainingVref-inputted sense channels. In an alternate embodiment, the drivelines of the touch sensor panel can concurrently be separately orsimultaneously stimulated with the stimulation signals Vstm_sc. Thesense channel output(s) can be captured (940). The measurement S can beobtained for the stimulated sense line j (945). For example, the outputfrom the sense channel connected to the stimulated sense line j and theoutputs from the k other sense channels connected to the k other senselines can be summed to obtain S. Alternatively, the outputs can beaveraged to obtain S.

A ratio T=S(j)/Zm(j) can be calculated (950). A body capacitance factorR can be calculated based on the ratio, as in Equation (5), for example(955). R can then be used to compensate for the body capacitance effectat a pixel. This method can be repeated for each stimulated sense line.

In some embodiments, R can be determined in a stray mode scan performedconcurrently with a normal mode scan. In some embodiments, R can bedetermined in a stray mode scan performed between every normal modescan. In some embodiments, R can be determined in a stray mode scanperformed less frequently, e.g., after a certain consecutive number ofnormal mode scans.

It is to be understood that determining a body capacitance effect is notlimited to the method of FIG. 9, but can include other methods capableof performing according to various embodiments. Although the method ofFIG. 9 shows the normal mode and the stray capacitance mode beingperformed separately, it is to be understood that the two modes can alsobe performed concurrently or that one mode can be performed one or moretimes followed by the other mode performed one or more times.

FIG. 10 illustrates exemplary circuitry of a touch sensitive device instray capacitance mode for measuring an effect of body capacitanceaccording to various embodiments. In the example of FIG. 10, touchsensor panel 124 can include drive lines 1001 (D0-D3) and sense lines1002 (S0-S3) that can form pixels 126. Transimpedence amplifiers 1009can connect to sense lines 1002 forming sense channels. In straycapacitance mode, the drive lines 1001 can connect to either Vcm orground. Amplifier 1009-a can be configured as a transmitter byconnecting a stimulation signal Vstm_sc to its non-inverting input.Amplifiers 1009-b, 1009-c, and 1009-d can be configured as receivers byconnecting a reference signal Vref to their non-inverting inputs.Amplifier 1009-a can stimulate sense line S0 with the same stimulationsignal Vstm_sc present on its non-inverting input. The output voltageVsc_tx from the amplifier 1009-a can be a function of the straycapacitance on the sense line S0.

In this example, when a user's hand contacts pixels D1,S0 and D2,S2, asignal charge can be transferred from stimulated sense line S0 throughthe finger contacting pixel D1,S0, the finger contacting pixel D2,S2,into sense line S2, via capacitances Cfd and Cfs. The amount of signalcharge transferred can be a function of the user's body capacitanceCbody. The output voltage Vsc_tx from the amplifier 1009-b (connected tosense line S2) can be a function of the amount of charge transferredthrough the user's hand and therefore a function of the user's bodycapacitance Cbody.

Normally, contacting the pixel D2,S2 can cause a reduction in signallevel at amplifier 1009-b. However, due to the charge transfer frompixel D1,S0 to pixel D2,S2, the signal can actually increase, hence, anegative pixel.

The outputs Vsc_tx and Vsc_rx can be measured (e.g., measurement Spreviously described) and used to determine the body capacitance factorR.

FIG. 11 illustrates an exemplary circuit in normal mode for detecting acapacitive touch according to various embodiments. In the example ofFIG. 11, transimpedence amplifier 1109 of a sense channel in receivesection 107 can receive a stable reference voltage Vref at thenon-inverting input of the amplifier and can receive a stimulationsignal Vstm run through a mutual capacitance Csig via touch sensor panel124 at the inverting input of the amplifier. The amplifier 1109 canoutput voltage Vout, which can be indicative of a touch at the panel.

FIG. 12 illustrates an exemplary circuit in stray capacitance mode formeasuring body capacitance according to various embodiments. In theexample of FIG. 12, transimpedence amplifier 1209 of a sense channel inreceive section 107 can receive a stimulation signal Vstm_sc at thenon-inverting input of the amplifier, while the drive lines can be heldat Vcm or ground. Therefore, Csig can become part of Cstray present atthe inverting input of the amplifier 1209. In some embodiments, thestimulation signal Vstm_sc can have an arbitrary wave shape, e.g., asine wave, a ramped square wave, etc., with or without envelope shaping.In stray capacitance mode, the output voltage Vsc can be a function ofCstray, Rfb, and the stimulation signal Vstm_sc frequency. If theamplifier 1209 operates in a linear operating range, the amplifier cankeep both inverting and non-inverting inputs at the same or similarvoltage levels. As such, the stimulation signal Vstm_sc can also bepresent on the sense line connecting to the inverting input and theamplifier 1209 can drive the sense line. In some embodiments, theamplitude and frequency of Vstm_sc can be programmable and the feedbackresistor Rfb can be programmable. Stimulation signal frequencies can beselected, e.g., so as to avoid environmental noise being injected intothe sense lines. Noise injection can occur, for example, through one ormore touching object, such as fingers, via a noise coupling capacitor(not shown). The amplifier 1209 can output voltage Vsc, which can beindicative of a touching user's body capacitance.

During the stray capacitance mode, the touch sensor panel can be scannedin steps, where each step can apply a stimulation signal Vstm_sc to oneor more of the sense channels and the output of the sense channels canbe captured. The number of steps can vary, generally up to the number ofsense channels, depending on the number of sense channel outputs deemedsufficient to measure the body capacitance effect. After all the stepsare completed, the outputs can be processed to determine the bodycapacitance effect. In some embodiments, each step can apply astimulation signal Vstm_sc to a different one of the sense channels andthe outputs of the sense channels can be captured. The output from asense channel can be a signal whose amplitude is the stimulation signalVstm amplitude scaled by the gain (e.g., √{square root over(1+(ω_(STM)·R_(fb)·C_(STRAY))²))} of the transimpedence amplifier. Insome embodiments, the gain value can be adjusted per step and per sensechannel. As such, a gain lookup table (LUT) can be stored in memory ofthe touch controller to access gain values to apply to the sensechannels during each step of the body capacitance mode scan. In someembodiments, the gain values can be scaled based on the frequency of thestimulation signal Vstm used during normal mode.

In some embodiments, all of the sense channels can connect to Vcm orground through the inverting inputs of their amplifiers, one of thesense channels can connect to Vstm_sc through the non-inverting input ofits amplifier, and the remainder of the sense channels can connect toVref through the non-inverting inputs of their amplifiers. As such, theone sense channel can become a drive channel and the remaining sensechannels can remain as sense channels that can receive output from thetouch sensor panel 124 for measuring an effect of body capacitance.

In some embodiments, multiple sense channels can connect to Vstm_scthrough their non-inverting inputs of their amplifiers to become drivechannels and the remaining sense channels can connect to Vref throughtheir non-inverting inputs of their amplifiers to remain sense channels.The Vstm_sc-inputted sense channels can alternate with the Vref-inputtedsense channels in a layout. Or the Vstm_sc-inputted sense channels canbe grouped together and the V-ref inputted sense channels can be groupedtogether in a layout. Other layouts can also be used, depending on theneeds of the device.

In some embodiments, the amount of charge that is transferred from aVstm_sc-inputted sense channel to a Vref-inputted sense channel can bedetected. Based on the amount of detected charge, a flag can be raisedto indicate a poor grounding condition. For example, a charge thresholdcould be set for each sense channel and compared to the detected charge.The detected charge exceeding the charge threshold for the Vref-inputtedsense channels can raise the flag to indicate poor grounding. The flagcan be used to trigger compensation of negative capacitance.

In some embodiments, the stray capacitance mode can be used to detect atouch condition. If a finger touches a sense line, a stray capacitanceof ΔCstray can be added to that sense line. Accordingly, after a scan ofthe touch sensor panel, the outputs of the sense channels can be summedand compared to a stray capacitance threshold. For example, the summedoutput exceeding the threshold can indicate a touch on the touch sensorpanel so that normal mode can be enabled, the drive and sense linesswitched to normal mode configuration, and the processor and othercomponents woken up to further process touch signals. Conversely, thesummed output below the threshold can indicate no touch on the touchsensor panel so that the processor and other components can remaininactive. This can be a power-saving measure.

FIG. 13 illustrates an exemplary method for measuring an effect of bodycapacitance according to various embodiments. In the example of FIG. 13,a stray capacitance mode can be enabled (1305). The non-inverting inputpins on one or more sense channel(s) can be switched from connections toVref to connection to Vstm_sc (1310). The drive signals outputted by thedrive channels to the touch sensor panel can be switched from Vstm toVcm (or ground) (1315). The touch sensor panel can be scanned togenerate touch signals that can be processed by the sense channels(1320). The processed touch signals outputted by the sense channels canbe captured (1325). Body capacitance factor R can be calculated (as inEquation (5)) based on, for example, the measurement Zm obtained fromthe sense channel outputs and the measurement S obtained from apreceding normal mode scan (1330). R can be stored for furtherprocessing. Normal mode can be enabled (1335).

FIG. 14 illustrates another exemplary circuit in stray capacitance modefor measuring body capacitance according to various embodiments. In theexample of FIG. 14, transimpedence amplifier 1409 of a sense channel canreceive a stimulation signal Vstm_sc at the non-inverting input of theamplifier while the drive lines can be stimulated with the stimulationsignal Vstm_sc, which can run through a mutual capacitance Csig viatouch sensor panel 124 at the inverting input of the amplifier. Theamplifier 1109 can output voltage Vsc, which can be indicative of a bodycapacitance.

When the drive lines are connected to Vcm (or to ground) as in FIG. 12,the capacitance Csig formed between the drive lines and a crossing senseline can contribute to the total stray capacitance Cstray on the senseline, which can be a function of Cs, the capacitance of the sense lineto ground; Csig, the mutual capacitance between the drive lines and thesense line; and the number of drive lines Nrows intersecting the senseline. When a user's finger touches a pixel on the sense line, the senseline capacitance can change based on the finger to sense capacitance Cfsand the user's body capacitance Cbody.

Therefore, the dynamic range utilization Dsc of a capacitancemeasurement at the sense line can be expressed as a function of Cstray,Cfs, Cbody, Csig, and Nrows as follows.

$\begin{matrix}{{D_{sc} = {k\; {\log \left( \frac{C_{fs}{}C_{body}}{C_{s} + {N_{ROWS} \cdot C_{sig}}} \right)}}},} & (8)\end{matrix}$

where k can be a constant, typically 20, associated with a voltage ratioor a power ratio. In some embodiments, the dynamic range utilization Dsccan be a function of a constant k′ (typically 10) times the log of asquared term, such as the square of the log term in Equation (8).

The numerator of Equation (8) can represent the capacitance change of agiven sense line due to an object, e.g., a finger, touching above thesense line. The denominator of Equation (8) can represent the totalsense line stray capacitance. The denominator term N_(ROWS)·C_(sig) canrepresent the mutual capacitance (Csig) contribution of the total numberNrows of pixels on a given sense line to the overall stray capacitanceof the sense line. For example, suppose Csig=1.3 pF, Cfs=1 pF, Cs=25 pF,Cbody>>Cfs, and Nrows=15. The dynamic component ΔCstray can undesirablybe −33 dB down from the full scale value of Vsc as shown in FIG. 12, forexample, resulting in lower signal-to-noise ratio. To increase thedynamic range utilization and thereby the signal-to-noise ratio, thedrive lines can be modulated with Vstm_sc, as shown in FIG. 14, forexample, such that the mutual capacitance Csig can be canceled out anddrop to zero. As such, the N_(ROWS)·C_(sig) term can drop out ofEquation (8) and the total dynamic range utilization Dsc of the sensechannel can become, for example,

$\begin{matrix}{D_{sc} = {k\; {{\log \left( \frac{C_{fs}{}C_{body}}{C_{s}} \right)}.}}} & (9)\end{matrix}$

Accordingly, in this example, the dynamic range utilization can improveapproximately 5 dB over the previous case. The amount of improvement candepend on the given application and can be more or less than 5 dB.

In some embodiments, all the drive channels can output Vstm_sc. In someembodiments, one or more of the drive channels can output Vstm_sc andthe remainder can output Vcm. In some embodiments, one of the sensechannels can connect to Vstm_sc through the non-inverting input of itsamplifier and the remaining sense channels can connect to Vref throughthe non-inverting inputs of their amplifiers. In some embodiments,multiple sense channels can connect to Vstm_sc through the non-invertinginputs of their amplifiers to become drive channels and the remainingsense channels can connect to Vref through the non-inverting inputs oftheir amplifiers to remain receive channels. The Vstm_sc-inputted sensechannels can alternate with the Vref-inputted sense channels in alayout. Or the Vstm_sc-inputted sense channels can be grouped togetherand the Vref-inputted sense channels can be grouped together in alayout. Other layouts can also be used, depending on the needs of thedevice.

FIG. 15 illustrates an exemplary method for measuring an effect of bodycapacitance according to various embodiments. In the example of FIG. 15,a stray capacitance mode can be enabled (1505). The non-inverting inputpins on one or more sense channel(s) can be switched from connections toVref to connection to Vstm_sc (1510). The drive channels can be switchedto connect to Vstm_sc. The touch sensor panel can be scanned to generatetouch signals that can be processed by the sense channels (1515). Theprocessed touch signals outputted by the sense channels can be captured(1520). Body capacitance factor R can be calculated (as in Equation (5))based on, for example, the measurement Zm obtained from the sensechannel outputs and the measurement S obtained from a preceding normalmode scan (1525). R can be stored for further processing. Normal modecan be enabled (1530).

FIG. 16 illustrates an exemplary circuit switchable between normal modeand stray capacitance mode according to various embodiments. In theexample of FIG. 16, switch 1603 can switch between a stimulation signalVstm_n or Vstm_p, a common voltage signal Vcm, or ground gnd duringnormal mode and between a stimulation signal Vstm_sc_p or Vstm_sc_n, acommon voltage signal Vcm, or ground gnd, during stray capacitance mode,outputted by the drive channels of the transmit section 114. Switch 1607can switch the non-inverting input of transimpedence amplifier 1609between a stimulation signal Vstm_sc_p or Vstm_sc_n, during straycapacitance mode, and a reference voltage signal Vref, during normalmode.

FIG. 17 illustrates an exemplary method for using a measured effect ofbody capacitance to compensate for negative capacitance according tovarious embodiments. In the example of FIG. 17, a normal mode can beenabled (1730). The sense and/or drive channel configurations of aprevious stray capacitance mode can be switch to their appropriateconfigurations during normal mode (1735). The touch sensor panel can bescanned to generate touch signals that can be processed by the sensechannels (1740). The processed touch signals outputted by the sensechannels can be captured (1745). Negative capacitance at the pixels canbe compensated for in the outputs (as in Equation (4)) based on apreviously determined body capacitance factor R (e.g., as illustrated inFIG. 9) (1750).

In some embodiments, the touch sensitive device can perform the straycapacitance mode and the normal mode concurrently, using a newlycalculated body capacitance factor R with the touch signals. In someembodiments, the touch sensitive device can switch between the straycapacitance mode and the normal mode for each scan, using the calculatedbody capacitance factor R with the touch signals from each normal modescan. In some embodiments, the device can switch to the straycapacitance mode after a certain number of normal mode scans, using thesame calculated body capacitance factor R with the touch signals frommultiple normal mode scans.

FIG. 18 illustrates an exemplary mobile telephone 1830 that can includetouch sensor panel 1824, display 1836, and other computing system blocksthat can measure an effect of body capacitance according to variousembodiments.

FIG. 19 illustrates an exemplary digital media player 1930 that caninclude touch sensor panel 1924, display 1936, and other computingsystem blocks that can measure an effect of body capacitance accordingto various embodiments.

FIG. 20 illustrates an exemplary personal computer 2030 that can includetouch sensor panel (trackpad) 2024, display 2036, and other computingsystem blocks that can measure an effect of body capacitance accordingto various embodiments.

The mobile telephone, media player, and personal computer of FIGS. 18through 20 can realize power savings, improved accuracy, faster speed,and more robustness by measuring a body capacitance effect according tovarious embodiments.

Although embodiments have been fully described with reference to theaccompanying drawings, it is to be noted that various changes andmodifications will become apparent to those skilled in the art. Suchchanges and modifications are to be understood as being included withinthe scope of the various embodiments as defined by the appended claims.

1. A circuit comprising: a sense channel having a switchable input; adrive channel associated with the sense channel; and switching circuitrycoupled to at least the sense channel and configured to operate thecircuit in a first mode for measuring a body capacitance effect of anobject proximate to the circuit by transmitting a stimulation signalthrough the input of the sense channel, and a second mode for measuringa touch of the object at the circuit by transmitting a reference signalthrough the input of the sense channel.
 2. The circuit of claim 1,wherein the first mode includes transmitting a common voltage signalfrom the drive channel.
 3. The circuit of claim 1, wherein the secondmode includes transmitting a second stimulation signal from the drivechannel.
 4. The circuit of claim 1, further comprising a processorconfigured to measure the body capacitance effect.
 5. The circuit ofclaim 1 incorporated into at least one of a mobile telephone, a digitalmedia player, or a personal computer.
 6. A circuit comprising: multiplesense channels, each sense channel having at least two inputs; multipledrive channels associated with the sense channels, each drive channelassociated with a first of the at least two inputs of a correspondingsense channel; and switching circuitry coupled to at least the sensechannels and configured to operate the circuit in a first mode formeasuring a body capacitance effect of an object proximate to thecircuit by transmitting a stimulation signal from each drive channel tothe first input of a corresponding sense channel and by transmitting thestimulation signal to a second of the at least two inputs of at leastone of the sense channels, and a second mode for measuring a touch ofthe object at the circuit by transmitting a reference voltage to thesecond input of the at least one of the sense channels.
 7. The circuitof claim 6, wherein the first mode includes transmitting the referencevoltage to the second input of the remainder of the sense channels. 8.The circuit of claim 6, further comprising: multiple sense lines, eachsense line associated with a corresponding sense channel, wherein thefirst mode includes the stimulation signal to the second input of the atleast one of the sense channels stimulating a corresponding sense line,the stimulated sense line injecting charge from a first circuit locationassociated with the stimulated sense line to a second circuit locationassociated with another sense line, and the another sense linetransmitting the injected charge to the first input of a correspondingsense channel for measurement.
 9. The circuit of claim 8, wherein theinjected charge measurement is associated with the body capacitanceeffect.
 10. The circuit of claim 6, wherein the second mode includestransmitting a second stimulation signal from each drive channel to thefirst input of a corresponding sense channel.
 11. The circuit of claim6, wherein the switching circuitry comprises a switch configured toswitch the second input of a sense channel based on the mode.
 12. Amethod comprising: enabling a mode for measuring a body capacitanceeffect in a touch sensor panel couplable to a sense channel and a drivechannel; switching the sense channel from a reference signal input to astimulation signal input; capturing outputs of the sense channelcomprising signals from the touch sensor panel indicative of the bodycapacitance of an object proximate to the panel; and measuring the bodycapacitance effect based on the captured outputs.
 13. The method ofclaim 12, further comprising: determining whether the captured signalsindicate poor object grounding; and generating an indicator based on thedetermination.
 14. The method of claim 12, further comprising:programmably scaling the stimulation signal input to the sense channel.15. The method of claim 12, further comprising: receiving at the drivechannel the stimulation signal input, the received drive channel inputconfigured to compensate for a capacitance formed at the panel.
 16. Themethod of claim 12, further comprising: transmitting the stimulationsignal input downstream of the sense channel, the transmittedstimulation signal configured to improve a dynamic range of the sensechannel.
 17. The method of claim 12, further comprising: subtracting thestimulation signal input from the captured outputs in order to at leastone of increase a dynamic range of the captured outputs or increase asignal-to-noise ratio of the captured outputs.
 18. The method of claim12, further comprising: subtracting the stimulation signal input fromthe captured outputs in order to reduce a dependency of the capturedoutputs on a frequency-dependent phase shift of the stimulation signalinput.
 19. A method comprising: obtaining a first capacitancemeasurement from a device during a first mode of the device; obtaining asecond capacitance measurement from the device during a second mode ofthe device; calculating a ratio between the first and second capacitancemeasurements; and calculating a capacitance factor based on thecalculated ratio.
 20. The method of claim 19, wherein obtaining thefirst capacitance measurement comprises: enabling the first mode of thedevice; capturing outputs from the device associated with a capacitancechange indicative of a object touch at the device; and obtaining thefirst capacitance measurement from the captured outputs.
 21. The methodof claim 19, wherein obtaining the second capacitance measurementcomprises: enabling the second mode of the device; capturing outputsfrom the device associated with a capacitance indicative of poorgrounding of an object proximate to the device; and obtaining the secondcapacitance measurement from the captured outputs.
 22. The method ofclaim 19, further comprising: applying the capacitance factor to outputsof the device to correct for poor grounding.
 23. A touch sensitivedevice comprising: a touch sensor panel configured to sense an objectproximate thereto; scan logic couplable to the touch sensor panel andconfigured to perform a first scan sequence on the panel to measure atouch by the object at the panel during a first mode and to perform asecond scan sequence on the panel to measure grounding of the objectduring a second mode; and a processor configured to compensate touchmeasurements according to grounding measurements.
 24. The device ofclaim 23, wherein the second scan sequence is further performed todetermine an occurrence of the touch by the object at the panel andwherein the device switches to the first mode if the touch is determinedto have occurred.
 25. The device of claim 23, wherein the scan logicconcurrently performs the first scan sequence and the second scansequence or the scan logic alternates between performing the first scansequence and the second scan sequence.